Cristina Antonanzas, the vice-secretary general of Spain's UGT labour union, said: "We cannot stigmatise women again, putting the focus on us as we have leave because we are on our period."
Ana Gomez, the head of the Asnala association of labour lawyers, said the measure may end up proving "negative" for women, adding: "It could limit their chance of being hired because it refers to the specific biological condition of women."
However, other experts have shown support for menstrual leave as a new landmark right for women.
"It is not about all working women having monthly leave if they menstruate, but about recognising that right for those who suffer incapacitating pain and who until now had to put up with it at work," said Carolina Vidal Lopez, the CCOO labour union's head of equality for women at work.
Government to foot cost of menstrual leave
Irene Montero, Spain's equality minister, pointed out that unlike other forms of absence, the cost of menstrual leave will be covered by the government, meaning it will not hit employers financially and also not depend on how long an employee has been making payments into the social security system.
Montero said after a cabinet meeting that the new law would be the first of its kind in Europe and "end the taboo" surrounding women's periods.
In the same reform package, which will now enter a consultation process before being presented to Spain's parliament, Madrid approved a change to abortion laws to allow 16- and 17-year-old girls to have the procedure without parental consent.
Abortion will also be a guaranteed service in any woman's nearest general hospital, while currently there are entire provinces in Spain that do not offer terminations as doctors have exercised their right to object on conscientious grounds.
Health centres will offer the morning-after pill free of charge, and research into male contraception methods will be boosted, Montero announced.
A plan to drop VAT on sanitary pads and tampons was left out of the draft and will be considered at a later date.